1. Field of the Invention
The present invention relates to a digital processing circuit for a video signal. More particularly, the present invention relates to a digital processing circuit for a video signal, utilized for example in a video tape recorder, in which a composite video signal is converted into digital signals so as to be recorded in a magnetic tape and the recorded digital signals are reproduced so as to be provided as the initial composite video signal.
2. Description of the Prior Art
In a video tape recorder and the like, in order to record a composite video signal in a digital form, it is necessary to sample and digitally process a composite video signal. One of the methods for digitally processing a composite video signal is well known as in "Sub-Nyquist Sampled PCM NTSC Color TV Signal Derived from Four Times the Color Subcarrier" published by J. P. Rossi in pages 218 to 221 in IEEE Conference Publication 166 (1978).
FIG. 1 is a schematic block diagram showing a conventional digital processing circuit. First, referring to FIG. 1, a conventional digital processing circuit will be described. A composite video signal comprising a luminance signal and a color signal is applied to an A/D converter 2 through an input terminal 1. To the A/D converter 2, a clock signal is supplied from a reference clock generator 4 making oscillation in response to a quartz crystal oscillator 3. The A/D converter 2 converts the composite video signal into a digital signal based on a clock signal. The signal converted into a digital signal is transmitted through a transmission line 5. The digital signal transmitted through the transmission line 5 is supplied to a D/A converter 6. To the D/A converter 6, a clock signal is supplied from a reference clock generator 8 making oscillation in response to a quartz crystal oscillator 7. This clock signal is in synchronism with the above described clock signal generated from the reference clock generator 3. The D/A converter 6 converts the transmitted digital signal into an analog signal based on the clock signal from the reference clock generator 8 and provides the analog signal from an output terminal 9. The clock signals provided from the reference clock generators 4 and 8 are generally selected to have frequency three times the color subcarrier frequency of a composite video signal.
In order to convert a composite video signal into a digital signal, it is necessary to apply sampling with a clock signal having frequency twice as large as the maximum frequency component included in the composite video signal, based on the sampling thereon. For example, in an NTSC system, the maximum frequency of a composite video signal is approximately 4.3 MHz of a luminance signal and therefore it is needed to apply sampling with frequency of more than 8.6 MHz. On the other hand, in order to decrease the quantization noise, it is desirable to apply sampling with a clock signal having frequency which is an integer multiple of a color subcarrier frequency. In a conventional system, a clock signal having sampling frequency (10.74 MHz) three times as large as the color subcarrier frequency was applied.
However, if the resolution is lowered and the maximum frequency of a luminance signal is as low as approximately 3 MHz, such frequency is of a level sufficiently suited for domestic use and in such case, the frequency of the color subcarrier becomes the maximum frequency included in a composite video signal, which makes it impossible to apply sampling with frequency lower than twice the color subcarrier frequency. As a result, it is needed to make an A/D converter 2 and a D/A converter 6 have high speed for conversion, which involves a disadvantage that formation of a circuit comes expensive. In addition, when a composite video signal is converted into a digital signal to be transmitted through a transmission line 5, a large number of signals cannot be multiplexed in a given frequency band.